Prevention of corrosion



Patented Nov. 1, 1938 I T E ii NT Fic PREVENTION OF CORROSION Herman A. Beekhuis, Jr., Petersburg, Va., as-

signor, by mesne assignments, to The Solvay Process Company, New York, N. Y., a corporation of New York No' Drawing.

Continuation of application Serial No. 572,446, October 31, 1931. This application October 23, 1936, Serial No. 107,153

' 3 Claims.

The present invention relates to methods of preventing corrosion of ferrous metals by ammonium nitrate solutions containing free ammonia.

In recent processes for the ammoniation of superphosphates, use is made of solutions containing large percentages of ammonium nitrate and also free ammonia. The provision of methods which reduce or prevent the corrosion of apparatus in which these solutions are handled constitutes a major problem in the commercial use of these solutions, since when ammonium nitrate-ammonia solutions, of the concentration used, are shipped in tank cars, rapid corrosion occurs and the tank soon is not usable. Also corrosion would occur in apparatus constructed of ferrous metal in which the solutions would be handled.

Passivation of the metal before contact with the solution would appear to be a possible method to prevent corrosion by solutions of ammonium nitrate which contain free ammonia. However, I have observed that passivated ferrous metals, though not actively corroded for a short period after immersion into ammonium nitrate-ammonia solutions, are thereafter rapidly corroded and that the short periods during which active corrosion does not occur are of erratic length.

It is an object of this invention to provide a process whereby the corrosion of ferrous metals .by the above described solutions, particularly by solutions containing about or more am- 'monium nitrate and 5% or more free ammonia, may be prevented for reliably long periods, so that it is commercially practicable to employ apparatus for these metals for the storage and transportation of such solutions.

It is a further object of this invention to provide solutions of ammonium nitrate containing free ammonia which do not corrode passivated ferrous metals for reliably long periods.

According to the present invention, the apparatus which may be a storage tank, a tank car, conveying pipe, measuring tank or the like, is constructed of ferrous metals such as cast. iron,

wrought iron, low carbon steels, for example structural steel, andalloy steels, for. example, one

permanganate, of a dichromate, or, in the case of a chrome steel, a solution of nitric acid. The apparatus so passivated is contacted with a corrosive solution containing ammonium nitrate, and ammonia, which solution may either be anhydrous or contain water. A chromate is incorporated in the solution of ammonium nitrate containing free ammonia which it is desired to contact with the ferrous metals. In the presence of such a solution the passivation of the metal is virtually permanent and does not break down after a short period, as would be the case in the absence of a chromate.

In order to incorporate a chromate in the solution of ammonium nitrate and ammonia, any suitable material may be added to the solution, such as chromic acid, a chromate or a dichromate. These materials, when dissolved in the ammonium nitrate solutions here involved, appear to be full equivalents, having equal solubility therein, on the basis of CrOs content, and being of equal merit in preventing destruction of the passivation. Thus, when chromic acid, sodium, potassium or ammonium chromate or sodium, potassium or ammonium dichromate are added to individual portions of a solution containing ammonium nitrate, ammonia and water in the proportions 2020, the following solubilities are found for each of these materials:

Weight, Suiting out temperature, 0. percent CrO;

is eifective without a chromate being added to the solution.

While any suitable strongly oxidizing agents may be used to effect the initial passivation of the ferrous metals, I prefer to employ as passivators a solution of 15% or less of chromic acid or of a My preferred range is from 0.02% to.

dichromate, of 1.5% or less potassium permanganate, and in the case of chrome alloy steels, a 10% solution of nitric acid.

My invention has specific application to solutions of ammonium nitrate containing free ammonia and water in which the ammonium nitrate constitutes about 50% or more of the solution (for example, 40% to 80%) and in which the ammonium nitrate is dissolved in 10% or stronger aqua ammonia, preferably in an aqua ammonia of about 25% or greater strength, due to the extremely high rates of corrosion exhibited by these solutions.

In order that my invention may be more clearly understood, the following examples, typical of preferred methods of procedure, are given by way of illustration:

Example I.-Structural steel containing, 0.20% carbon is pickled with dilute hydrochloric or. sulfurioacid in the usual manner to remove mill scale and is immersed in or washed with a 10% solution of sodium dichromate of a temperature of 50 to 100 C. for a period of thirty minutes. The steel is then contacted with a solution of the following composition:

Parts by weight NHNOPs 60 Na2Cr2Om2H20 0.1

In this solution the amount of chromate calculated as CrOs is 0.07%.

Steel which has been treated by the above process has shown no appreciable loss by weight due to corrosion after 10 months exposure to the solution.

Example II.+Wroughtiron is pickled as in Example I and is then passivated with a hot 1% solution of potassium permanganate. The wrought iron is then contacted with a solution of the following composition:

Parts by weight NH4NO3 75 NHs 25 K2CIO4 0. 1

Parts by weight NH4NO 60 NHa- 20 H20 20 K2C1O4 0.1

In this solution the amount of chromate calculated as CrOa is 0.05%. A chrome steel so passivated has shown excellent resistance to corrosion over a long extended period.

Example IV.--A steel tank in which the cylindrical section is fabricated by welding and the top and bottom are bolted on is cleaned with an acid and passivated with an aqueous sodium chromate solution and then filled with an ammonium nitrate-ammonia-water solution in the proportions 60-20-20 containing about 0.1 per cent sodium dichromate. In this solution the amount of chromate calculated as CrOa is 0.07%. No corrosion has occurred in a tank so treated after the period of 1 years. During this period the tank has been submitted to severe conditions; at times the tank has been left with almost all the solution removed and at other times an agitator has been operated in the solution in the tank, the flow of liquor generated being allowed to impinge against the walls of the tank.

The following theory is offered as a possible explanation of the eifectlveness of chromates for preventing corrosion of ferrous metals by ammoniacal ammonium nitrate solutions.

It appears from my observations of the action of numerous salt solutions on ferrous metals that the chemical reaction which causes rapid corrosion of ferrous metals by ammoniacal ammonium nitrate solutions is peculiar to nitrate solutions and ferrous metals and causes rapid corrosion of these metals only in the case of the ammoniacal solutions. Thus, in the case of acid or neutral solutions of ammonium nitrate, hydrogen is evolved by the corrosion of ferrous metal by the solutions. In the case of corrosion by ammoniacal ammonium nitrate solutions, instead of detecting the evolution of hydrogen, I have observed that ammonia is formed by reactions taking place during corrosion of the metal by the solution. This invention is based on the discovery that under the conditions encountered in the corrosion of ferrous metals by ammoniacal solutions of ammonium nitrate, the presence of a small amount of a chromate in the solution serves to prevent attack of previously passivated ferrous metals. This action appears specific to the chromates since hypochlorites, for examples, have been tried and appear not effective for the purpose of this invention. Ferrous compounds are also formed by the corrosion of ferrous metals by ammonium nitrate-ammonia solutions, and the solubility of these ferrous compounds appears to be high in ammoniacal solutions as compared with neutral solutions. It appears that the presence of ferrous ions in solution is accompanied by a rapid corrosion of the ferrous metal. The chromate incorporated in the solution may act at least in part to prevent solution of the ferrous compounds in the solution and thereby prevent corrosion of the metal.

This application is a continuation of my copending application Serial No. 572,446, filed October 31, 1931.

I claim:

1. The process for preventing corrosion of a ferrous metal by solutions of ammonium nitrate in about 25% or stronger aqua ammonia and containing about 50% or more of ammonium nitrate, which comprises treating said ferrous metal with a strongly oxidizing agent from the group consisting of chromic acid, a permanganate and a dichromate, incorporating a chromate in said ammonium nitrate-ammonia solution, and contacting said solution with said ferrous metal.

2. A method of inhibiting the corrosion of steel by the action of a solution containing from about 40% to 80% of ammonium nitrate together with excess ammonia which comprises pickling the steel in acid solution to remove rust and'scale therefrom, treating said steel with a 10% solution treating said. metal with about a 10% solution of sodium dichromate and subjecting the thus treated metal to the action of said ammoniacal solution of ammonium nitrate containing about 0.02% to 0.25% by weight of sodium dichromate. 5

HERMAN A. BEEKHUIS, JR. 

